1. Field of the Invention
The present invention comprises a positive-displacement pump having in the example described, four lobes. Such pumps may be used as pumps, motors, parts of generators, and a wide variety of uses known to the prior art.
2. Description of the Prior Art
The prior art includes many positive-displacement pumps, a substantial number of which comprise a matched pair of multilobed impellers housed in a pump chamber, including inlet and outlet ports located on opposite sides of the chamber. Typically, the impellers are affixed to parallel shafts which are in turn coupled to timing gears and drive means. In operation, the drive means rotate the impellers in opposite directions, somewhat out of phase, and the impellers form sealed compartments between their lobes and the adjacent interior pump chamber wall, thus substantially enclosing the fluid to be pumped and transporting it from the inlet to the outlet side of the pump chamber. Substantially all the fluid is prevented from being returned from the outlet side to the inlet side through the center of the meshing impellers because of the close proximity of the imposing parts and the precise clearance which is maintained by the timing gears which prevent contact between the counter-rotating impellers. In the past, many attempts have been made to design pump impellers for such pumps to overcome several shortcomings common to the generally accepted designs. Among these shortcomings are low volumetric efficiency, vibration, wear, etcetera. Volumetric efficiency may be defined as that part of the effective displacement of the pump which actually transports the fluid to be pumped. Any difference between the actual volume pumped and the effective pump displacement is regarded as a loss of pumping volumetric efficiency and a reduction of such loss is desirable. A further loss of pumping efficiency occurs due to internal pressures which develop during rotation of the impellers. Some of the fluid being transported slips back to the low pressure or inlet side of the pump chamber from the high pressure or outlet side of the pump chamber by flowing through and past whatever clearance exists between impeller tips and the adjacent pump chamber wall or the adjacent portion of the imposing impeller (between the two impellers). Other losses which reduce efficiency include friction caused by the drag of the fluid between walls and impellers, inertia of the moving parts, and diminished flow rate caused by fluid turbulence.
Rotary pumps designed for movement and compression of gases are commonly referred to in the trade as blowers, super chargers or compressors. The name usually indicates the application. Blowers and super chargers generally utilize two-lobed involute impellers which offer maximum displacement relative to pump chamber dimensions, thereby providing means of moving large volumes of gases at low pressures. Some blowers and super chargers use three-lobed impellers, especially where it is necessary to develop higher pressures and operate at higher velocities than is typical with two-lobed impellers. Compressors use two, three or even more lobes to comply with various specifications. Blowers and super chargers ordinarily operate dry and depend upon the close clearances between the moving parts to form pneumatic seals. Compressors often use wetting agents such as water or oil sprays to aid in maintaining effective pneumatic seals between moving parts. Generally the degree of compression or vacuum developed by such pumps is a function of effective sealing properties.
The present invention differs from the prior art in ways which increase operational efficiency, and useful life and lower the amount of energy required and fabrication costs for the following reasons, among others.
An impeller with four-lobed involute structure has several advantages over impellers with different numbers of lobes. Generally two-lobed involute impellers occupy 50 percent of the volume of pump chambers in which they operate, three-lobed impellers occupy 46 percent of the volume, but four-lobed involute impellers occupy only 41 percent of the volume of the pump chambers in which they operate. Accordingly, there is a substantial increase in effective displacement possible by using four-lobed involute impellers. Impellers having five or more involute lobes occupy more than 50 percent of the volume of pump chambers in which they operate because the lobes of such impellers must be shorter than the lobes of two, three or four-lobed impellers, thereby taking up a higher percentage of the volume inside the pump chamber. Multilobed impellers of the involute type may even approximate the appearance of actual involute gear teeth and because of their large bulk compared to the small space between lobes, they are impractical for use in pumping gases although they are often of value in pumping some other fluids.
Accordingly, four-lobed impeller pumps pump a greater volume per revolution through the pump chamber because of the increase in volumetric efficiency compared to the physical size of the interior of the pump chamber. There is also, of course, an increase in the volume pumped as compared to the mass of the impellers which results in higher horsepower efficiency, since more fluid is pumped compared to the power consumed in turning the non-contributing mass. The power loss in rotating impellers of greater weight such as two or three-lobed impellers cannot be offset by lightening the weight of such impellers by removing much of the bulk of the internal mass of such impellers because such action does not increase the effective displacement of the pumps using impellers of such construction.
Extensive testing has demonstrated that the most critical area of sealing is located where the impellers mesh with one another (where the tip of one impeller approaches the space between the tips of the adjoining impeller). Any excessive amount of clearance between the tips of the one impeller and the curved inner surface between the lobes of the opposite impeller results in a serious degree of fluid leakage back from the outlet to the inlet and greatly diminishes the pumps ability to compress gases, form a vacuum, or pump a fluid. This has been accomplished in the prior art by attempting to minimize the clearances. This problem is complicated because the shape of the cross-sectional area formed between the two impellers varies as a function of the rotation of the lobes and a corresponding orientation of the lobes and other surfaces of the two impellers. The problem is even more complicated, because not only does the volume change, but the change in volume has a tendency to change the internal pressures in the fluid and can be a source of severe vibration which can cause pump failure or inefficiency or wear, among other problems. Accordingly, external timing gears are frequently used in order to control the rotation freely and smoothly at high speeds. Such running gears must provide a minimum of backlash or running clearance between meshing teeth. However, in spite of adequate lubrication, after a period of rotation, such clearance will increase due to wear caused by sliding and rolling friction common to all meshing gears under load. In the prior art pumps, this causes the tracking corners of the impeller ends to strike the opposing impeller and thus cause noise and wear, deterioration of volumetric efficiency, decreased pressure and ultimate pump failure. The prior art discloses no designs which overcome this problem which prior to the present invention is almost seen to be inherent in the nature of a pump. The present invention, as will be explained later, includes an automatic clearance adjustment which does not affect the impeller's ability to accurately track and maintain proper clearance between its tips and the adjacent pump chamber walls. Because of this automatic clearance adjustment, a pump according to the present design will continue to function close to maximum efficiency in spite of normal gear wear. A pump according to the present invention does not require costly maintenance ordinarily expected of rotary lobed pumps of the prior art for correction, for wear caused by use.
Tests have shown that pumps pursuant to the present invention reduce the inefficiencies of prior art pumps by approximately 50 percent or more.
Additional advantages of four-lobed involute impellers compared to impellers of fewer lobes include a reduction in amplitude of pulsations caused by the release of compartments of fluid to the outlet chamber. Such pulsations are normally not of any consequence where the fluid pumped is a compressible gas, but they cause considerable vibration when liquids are pumped. Minimizing of these vibrations reduces excessive wear of gears and bearings caused by high amplitude pressure pulsations. The particular design of the present invention, particularly the way the tips mesh with the space between the lobes of the opposite impeller reduces such pulsations.
A further advantage of the present four-lobed design over two or three-lobed types is that a greater mass of the impellers is closer to the impeller center, resulting in a stronger structure and one requiring less energy to accelerate to operational velocities.
A four-lobed design is also superior for both gas and liquid pumps in its ability to develop and maintain adequate operational pressure. This inherent advantage is further improved by the present invention design since more points of near sealing are present to minimize the leakage of the fluid from the high pressure outlet side of a pump according to the present design to the low pressure inlet side, thereby resulting in a higher useful pressure being developed. The fact that with the present design, a greater part of the impeller mass is located closer to its center, the area of its lowest velocity, makes a structure according to the present invention inherently stronger and permits higher speed than possible with two or three-lobed designs otherwise structurally equivalent to the present invention.
Typically prior art pumps have problems with the depositing of materials from the fluid pumped such as dissolved salts, etcetera. A further advantage of the present invention is its self-cleaning capability since the leading sharp edges of the impeller tips continually and closely track the contours of the pump chamber walls in which they revolve, thereby effectively scraping off any deposits of solid which accumulate on such surfaces. Such scraping occurs on an incremental basis with extremely small amounts being scraped off during operation without any substantial loss of efficiency. In fact, the deposits normal to pumps utilizing fluid maintain the clearances of a pump according to the present design at or near the most efficient possible distances.
Pumps according to the present invention pump water with a volumetric efficiency of 90 percent at low head pressures and develop hydraulic horsepower efficiencies in excess of 85 percent. The same pump, used as a motor, can accelerate from a standing start to 3,000 RPM in two seconds with a driving fluid of air at 70.degree. Farenheit and 2 psi. This appears to make a pump of the present design capable of doing work presently done by gas turbines with greater efficiency and a substantially smaller manufacturing cost and replacement and maintenance cost. Pumps according to the present design require approximately half the energy required by centrifugal pumps for equivalent fluid movement.
Like most other pumps, the present design benefits by expansion of capacity and it appears that since the preceding efficiencies were obtained by a pump of 18.75 cubic inch displacement, larger pumps may reach a hydraulic horsepower efficiency in excess of 95 percent.
Many prior art designs cannot operate with all fluids. The present invention can operate and pump all fluids which do not have characteristics inherently capable of destroying the structure of the pump (such as extremely high temperature which would melt the pump, etcetera).
Certain prior art pumps are self-priming. These prior art pumps lose their self-priming capabilities because the self-priming capability is inherently a function of physical contact within the pump chamber. A pump according to the present invention does not lose its self-priming capability because it does not lose tolerance after normal timing gear wear.